1. Field of the Invention
The present invention relates to coaxial-cable communications systems. More particularly, the present invention relates to coaxial-cable communications systems employing single and multiple sinewave modulation techniques, and apparatus for modulating and demodulating information according to the modulation techniques in coaxial-cable communications systems employing the modulation techniques.
2. The Prior Art
Digital data is often transferred from point to point by exploiting one or more of the three characteristic properties of an AC signal: Amplitude, Frequency and Phase.
Some of the modulation methods using the amplitude property of carriers are OOK (On-Off Keying) and common AM (amplitude modulation). In OOK a data bit is represented by the presence or absence of a carrier (full amplitude or zero amplitude). In AM data bits are represented by a difference in the relative amplitude of the carrier or by using different frequency tones representing the digital data to amplitude modulate a carrier.
Typical modulation methods using the frequency property are FSK (Frequency Shift Keying) and FM (frequency modulation). With FSK the binary states of digital data are represented with abrupt frequency changes between two predetermined fixed frequencies. In FM data bits are represented by a difference in the relative frequency of the carrier or by using different tones representing the digital data to frequency modulate a carrier.
Phase modulation is also a common modulation method also but when used alone is difficult to distinguish from FM. Recently more sophisticated modulation methods have evolved and have been exploited. As an example a relatively new and popular modulation method is QAM (Quadrature Amplitude Modulation), which uses a combination of amplitude and phase modulation. There are several different variations of QAM depending on the application.
Each of these modulation methods is viable, proven and used in appropriate communications applications. A few undesirable characteristics common to all of these modulation methods is they all require several cycles to transmit one bit and in doing so generate significant sidebands. These sidebands are necessary to extract the information from the carrier and take up significant bandwidth in the communications channel, requiring significant spacing between adjacent signals.